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Lactate is recognized as a crucial signalling molecule within the tumor microenvironment, where it shapes immune responses by modulating various cell populations, including T cells and macrophages. However, its effect on natural killer (NK) cells, key effectors of early antitumor immunity, remains poorly understood. This study investigates how intratumoral lactate accumulation affects NK cell function in breast cancer, a neoplasm characterized by elevated glycolytic flux. An in-silico analysis of 882 breast cancer patients revealed that high lactate metabolism is inversely correlated with NK cell activation genes and is associated with poor prognosis. To corroborate these findings, NK cells from healthy donors were cultured under lactate-rich or control conditions. Lactate exposure impaired NK cell proliferation, downregulated activation markers and cytotoxic molecules, disrupted mitochondrial bioenergetics, and induced lipid accumulation, as demonstrated by flow cytometry, metabolic profiling, and Raman spectroscopy. Functional assays using microfluidic devices and degranulation tests revealed that lactate-exposed NK cells exhibited reduced chemotaxis and diminished cytotoxicity against MCF-7 and MDA-MB-231 breast cancer spheroids, accompanied by decreased CXCL9 and CXCL10 production. Pharmacologic inhibition of lactate transport, via Syrosingopine or MSC-4381 and AZD3965 combination, restored NK cell cytotoxicity in tumor co-cultures, as shown by increased NK cell degranulation, caspase-3/7-mediated tumor apoptosis, and spheroid shrinkage. Finally, GPR81 deletion mirrored these effects, enhancing NK cell activity. These findings identify lactate as a driver of NK cell suppression and highlight lactate transport and receptor targeting as a strategy to enhance NK cell-based immunotherapies in breast cancer and other lactate-rich tumors.